1. Field of the Invention
This invention relates in general to a fuel injection system, and more particularly to a structure for controlling the residual pressure in a fuel injection pump.
2. Description of the Related Art
As is well-known in the art, an in-line fuel injection pump for use in a diesel engine has a structure for feeding fuel under pressure. As shown in FIGS. 10 and 11 of the accompanying drawings, the inline fuel injection pump A includes a plunger D which is made to slide in a plunger barrel by a cam B. The plunger D feeds fuel to an injection pipe when the pressure of fuel becomes large enough to open a delivery valve E. Then, when the pressure of fuel in the oil pipe is large enough to open an injection nozzle connected to the injection pipe, fuel is injected into an engine cylinder.
On the other hand, when the plunger D moves upwardly and an opening F of the plunger D communicates with a port G of a sleeve C1, the pressure in a pressurization chamber H is reduced.
Referring to FIG. 11, the delivery valve E is closed by the pressure of its own return spring E1, thereby preventing reverse flow of fuel to the fuel injection pump A from the injection nozzle, increasing the capacity of the delivery valve chamber while the delivery valve is in its return stroke so as to lower the pressure in the delivery valve chamber, and lowering the pressure in the injection pipe connected to the valve chamber.
Fuel is instantaneously injected into the engine cylinder so as to prevent fuel from dripping into the cylinder after injection.
Fuel is confined in the injection pipe after it is injected into the cylinder. Therefore, a part of pressure applied to fuel is retained in the injection pipe as residual pressure.
The residual pressure represents a static pressure that exists when the pressurizing process is stabilized after one fuel injection process, and varies with engine operating conditions to determine factors such as an engine speed and the quantity of fuel injection.
The residual pressure is reduced when the quantity of injected fuel is larger than the quantity of fuel under pressure fed by the plunger to the delivery chamber. Otherwise, the residual pressued is increased.
When the residual pressure varies, transmission of the pressure for next fuel injection also varies in the injection pipe, so that the injection pressure will be changed accordingly.
When the return spring of the delivery valve has a preset constant pressure, variation of the residual pressure would cause differences in the pressure lowered during the return of the delivery valve. Fuel becomes foamy due to the reduction of the pressure in the injection pipe. Unfortunately, sometimes fuel is so extensively foamy that cavitation erosion will be remarkably caused when fuel foams are destroyed under pressure.
When the engine speed is low, the plunger moves so slowly to adversely affect the injection pressure. The lower the injection pressure, the less effectively fuel is changed to mist of fine fuel particles. Such fuel tends to produce a lot of particulates in an exhaust gas and soot.
This phenomenon is shown in FIGS. 12 and 13. FIG. 12 shows the relationship between a particulate diameter and injection pressure. FIG. 13 shows the quantity of particulates depending upon the injection pressure.
There has been proposed a structure to increase the pressure for opening the delivery valve regardless of the residual pressure varying depending upon the injection characteristics which are set according to the engine operating status. Then, the pressure applied to fuel is kept in the injection pipe so that fuel is injected under that pressure when the injection nozzle is opened.
Such a structure however requires an additional member such as an oil path for keeping the pressure, and a pump in the fuel chamber to keep the pressure, which makes the fuel injection system more complicated.
Further, there is a proposal in which a nonreturn valve is disposed to feed fuel to the delivery valve chamber so as to preliminarily pressurize the chamber when the residual pressure is lowered. In such a structure, fuel has to flow only in one direction in the delivery valve chamber.
Therefore, it is impossible not only to adjust the pressure as desired in the delivery valve chamber but also to increase the injection pressure depending upon the engine speed. This implies that a high pressure cannot be realized for the fuel injection in an engine speed range where fuel has to be extensively pressurized to decrease particulates in the exhaust gas.